This invention relates in general to internal combustion engines of the multi-spark plug type and, more particularly, to a method and apparatus for controlling ignition spark timing for such engines.
As is well known in the art, it has heretofore been proposed to have an internal combustion engine equipped with a plurality of spark plugs adapted to provide a plurality of flames which propagate from a circumference of a wall of a combustion chamber toward a center portion thereof. In this type of engine, the flame propagation distance is less than that of a prior art internal combustion engine of the type having a single spark plug and, therefore, it is possible to effect rapid combustion of air-fuel mixture in the combustion chamber. Accordingly, it makes it possible to increase the exhaust gas recirculation rate up to 40% without sacrificing the performance efficiency of the engine. Another advantage is that the concentration of nitrogen oxides in engine exhaust gases can be remarkably decreased. In a known internal combustion engine of the multi-spark plug type, it has been a usual practice to energize the spark plugs at the same ignition timing. Since, however, the combustion state in the combustion chamber varies in dependence on the state of air-fuel mixture supplied into the combustion chamber and operating conditions of the engine etc. Thus, it is desired that the spark plugs be energized at various ignition timings in dependence on the various factors.